Ultrasonic diagnostic apparatuses for forming a three-dimensional ultrasonic image have been put into practice. Such an ultrasonic diagnostic apparatus comprises an array transducer unit having a plurality of transducer elements for effecting electronic scanning with an ultrasonic beam, a mechanical scanning mechanism for moving the array transducer unit for mechanical scanning, and so on. With the above-described structure, an ultrasonic diagnostic apparatus creates a three-dimensional echo data acquisition space which is composed of a plurality of scanning planes, thus forming a three-dimensional ultrasonic image (for example, a projection image) based on a large number of echo data acquired within the three-dimensional echo data acquisition space. The scanning plane is formed by electronic scanning of the ultrasonic beams. Namely, the scanning plane is composed of a plurality of ultrasonic beams (sound rays). Accordingly, the three-dimensional echo data acquisition space is a collection of ultrasonic beams (an ultrasonic beam array). On the other hand, other types of ultrasonic diagnostic apparatuses which form a three-dimensional echo data acquisition space using a two-dimensional (2D) array transducer which includes a plurality of transducer elements arranged two-dimensionally have also been proposed. These apparatuses create a three-dimensional echo data acquisition space by two-dimensional electronic scanning of ultrasonic beams (without moving the transducers for mechanical scanning).
In recent years, ultrasonic diagnostic apparatuses for forming an image by using echoes reflected from an ultrasonic contrast agent which has been injected into a tissue (for example, a blood vessel) of a living body have also been put to use. Normally, echoes from blood are weaker than echoes from tissue, whereas echoes from an ultrasonic contrast agent are relatively strong. These apparatuses make use of this characteristic to form an image of a blood vessel. Specifically, an ultrasonic contrast agent is composed of a very large number of microbubbles (very small bubbles having a predetermined structure). When the ultrasonic wave reaches the microbubbles, the microbubbles destruct or disappear. At the same time, reflected waves which are distorted (echoes) are generated. Using the basic wave components or higher harmonic wave components of these echoes, an ultrasonic image is formed.
It is possible to clearly visualize the existence or behavior of an ultrasonic contrast agent by comparing two items of echo data which are acquired before and after destruction or disappearance of the microbubbles forming the ultrasonic contrast agent, or by comparing two items of echo data which are acquired before and after movement of the ultrasonic contrast agent.
In general, an ultrasonic contrast agent is continuously injected into tissue of a living body for a determined period once the injection has started. When two items of data (echo data or pixel data) which are acquired at the same beam address are compared, it is difficult to accurately form an image of the ultrasonic contrast agent if a time interval between the acquisition of these two items of data is too long. For example, assume that at a certain local region within a blood vessel, the microbubbles of the ultrasonic contrast agent destruct or disappear at the first irradiation of ultrasonic waves, and the second irradiation of the ultrasonic waves is not performed immediately after the first irradiation. Namely, assume that the second ultrasonic irradiation is not performed until a sufficient amount of the ultrasonic contrast agent is supplied to that local region through the blood flow. In such a case, there is not a significant difference between the data obtained from the first irradiation and data obtained from the second irradiation.
Further explanation will be given. Conventionally, when the three-dimensional echo data acquisition space is formed, the scanning plane is scanned (mechanically, for example) at a fixed rate, as described above. In such scanning, one scanning operation of the scanning plane requires, for example, one second. Accordingly, when the scanning operation of the scanning plane is carried out twice successively, the time interval between the two items of data acquired at the same beam address by these two scanning operations, results in, for example, one second. This makes it difficult to detect an instantaneous phenomenon by comparing the two items of data.